Q&A.

Why would snapdragons, full of buds, suddenly wilt?

Q. Why would my perfectly healthy double-flowered snapdragons suddenly wilt? I pinched them back earlier to encourage multiple stems and all the buds were ready to open. I am so disappointed.

--Leigh Carruthers, Evanston

A. Many plants, including annuals with very shallow root systems, exhibited sudden wilt symptoms during the wet weeks of May and June. When soil is flooded or saturated for more than a few days, root injury or death ensues because roots cannot perform basic, necessary functions, such as taking up oxygen and nutrients.

Even though there is more than enough water in the soil, the roots are unable to absorb it and the plant reacts by wilting. In stressful conditions like this, opportunistic pathogens that advance root rot and other soil-borne fungi take hold and plant tissue begins to die.

Remove all infected plants and refrain from planting until the soil is dry. Avoid planting in low areas and try to raise the level of the garden bed, if possible. Promote good drainage by incorporating organic matter and be quite sure your new plants are disease-free before adding them to your garden.

Q. Is it possible to keep a weeping Katsura tree pruned so that it remains under its present 10 feet?

--Mary Inciardi, Clarendon Hills

A. Cercidiphyllum japonicum `Pendulum' is a medium-sized weeping tree that grows 15 to 25 feet high and sometimes as wide. This is not a tree for small, confined spaces as its graceful arching branches demand adequate room to grow upward and outward. It is virtually impossible to prevent this tree from growing over 10 feet high.

Pruning back the top of a tree is tantamount to destroying its natural habit, and in some cases, destroying the tree. The main trunk of the weeping Katsura will continue to grow taller, elevating all the horizontal, weeping branches, until it achieves its full height. It is possible to prune smaller weeping side branches that spring from the large weeping branches.

Q. The lower leaves of our magnolia tree as well as the ivy on the ground below it are covered with a clear sticky substance. What could this be?

--Amy Spungen, Highland Park

A. When certain mites or insects, like aphids or scale, feed on plant foliage, they excrete a clear liquid known as honeydew, which often goes unnoticed until it becomes covered with a sooty black mold. The mold is not a threat to the plants, nor is the honeydew, but the insects can be.

Magnolia scale is one of the largest soft-scale insects. Look for large waxy "bumps" on twigs and stems of the plant. The immobile scales suck plant juices causing leaves and branches to die back. The most effective treatment targets the juvenile crawler scales that move rapidly about the tree looking for a place to settle down before they, too, develop a protective waxy dome-shaped coating. Magnolia scale crawlers are usually active in mid-September, a time when they are most vulnerable to insecticidal controls. At that time, spray your tree thoroughly with a lightweight horticultural oil, an insecticidal soap or a product containing acephate. Multiple applications may be necessary if the problem is quite advanced.